The atomic number of Uranium is 92 and its atomic mass is 238. How many neutrons does the most common isotope of Uranium have?
Interestingly enough, the atomic mass of this isotope is given to you as a whole number.
As you know, an isotope's mass number tells you how many protons and neutrons it contains in its nucleus. If you take
#color(blue)("mass number" = A = Z + "no. of neutrons")#
Well, if uranium's atomic number is equal to
#color(blue)("no. of neutrons" = A - Z)#
#"no. of neutrons" = 238 - 146 = color(green)(146)#
The same approach can be applied when the atomic mass is not a whole number. For example, the atomic mass of uranium is actually
In such cases, you need to round the atomic mass to the nearest integer value. This will give you the mass number of the element's most abundant isotope.
As you can see here,
Uranium-238, which is the most common isotope of uranium, contains
#146#neutrons in its nucleus